20. Which one of the following amino acids has the highest probability to be found on the surface of a typical globular protein in aqueous environment?
(A) Ala
(B) Val
(C) Arg
(D) Ile
Globular proteins fold to position hydrophilic amino acids on their surface in aqueous environments, maximizing water interactions for solubility. Among the options, arginine emerges as the clear choice due to its charged, polar side chain. This principle drives protein stability in water-based cellular conditions.
Option Analysis
Globular proteins bury hydrophobic residues inside and expose hydrophilic ones to solvent.
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Alanine (Ala): Small nonpolar side chain (-CH₃); prefers interior burial, low surface probability.
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Valine (Val): Branched nonpolar side chain (-CH(CH₃)₂); highly hydrophobic, typically internal.
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Arginine (Arg): Positively charged guanidino group; highly hydrophilic, forms H-bonds/salt bridges with water.
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Isoleucine (Ile): Bulky nonpolar side chain (-CH(CH₃)CH₂CH₃); strongly hydrophobic, favors core.
Correct Answer
Arginine (C) has the highest surface probability. Its basic side chain (pKa ~12.5) remains protonated in aqueous physiological pH, enabling strong hydration shell interactions.
Hydrophobicity Scales
Surface exposure correlates inversely with hydrophobicity scales (e.g., Kyte-Doolittle).
| Amino Acid | Hydrophobicity (KD Scale) | Surface Preference |
|---|---|---|
| Ala | 1.8 (mild hydrophobic) | Low |
| Val | 4.2 (hydrophobic) | Interior |
| Arg | -4.5 (hydrophilic) | High |
| Ile | 4.5 (very hydrophobic) | Interior |
Charged residues like Arg dominate ~30% of protein surfaces in water.
